With increasing public concerns on our environment and more stringent environmental legislations, excess sludge disposal involved biosolids management has been becoming an ever-tough challenge in developed countries such as Canada, USA, EU countries and Japan. From the concept of cleaner production and the vision of sustainable development, great effort should be made to develop some new process that can not only achieve satisfactory nutrient removal but also produce less or more ideally, zero excess sludge.
Excess sludge is often generated in the process of biological treating of wastewaters containing excessive nutrients such as chemical oxygen demand (COD), suspended solids and total nitrogen in the form of proteins and ammonia. Under aerobic conditions, carbonous nutrient can be easily oxidized to carbon dioxide and water while the biological removal of total nitrogen is subject to alternate nitrifying and denitrifying process. During the aerobic nitrification, primarily the ammonia oxidizing bacteria firstly convert ammonia to nitrite and the nitrate oxidizing bacteria further oxidize nitrite to nitrate. During the anoxic denitrification (preferably dissolved oxygen less than 0.5 mg/L), nitrate is converted to nitrogen gas by a group of denitrifying bacteria. In practice, nitrification and denitrification may be engineered as either two compartments within a single tank (i.e., aerobic zone and anoxic zone) or two separate tanks (i.e., aerobic tank and anoxic tank) connected by a recycle line. The final treated water is subsequently collected in the form of supernatant after gravity sedimentation, a proven less effective unit process for solid-liquid separation. In order to improve the quality of treated water, ultraporous or microporous membranes have been recently submerged in the aerobic compartment or the aerobic tank with a vacuum suction applied to one side of membranes. For those crossflow inside-out membranes, however, a side-stream design may be a more viable option in the treatment of various wastewaters. In both cases, clean water permeates out of membrane walls but impurities such as bacteria and suspended solids are confined within the bioreactor. To mitigate fouling tendency, membranes are usually kept awash by scouring air bubbles for submerged membrane bioreactors or high-speed circulation for side-stream installations. To maintain a desired sludge retention time (SRT), certain amount of excess sludge has to be wasted continuously or intermittently, depending on actual operational requirement of process design although the excess sludge itself is not ‘excessive’ from the microbiological metabolism point of view.
U.S. Pat. No. US6,616,843 Dl describes a single tank membrane bioreactor in which both aerobic and anoxic biodegradations prevail alternately at an average SRT between 10 days and 30 days. According to the invention, nutrients removal in terms of COD and total nitrogen is proven satisfactory during the seven months of operation with a feed of actual municipal wastewater. Two obvious drawbacks are (a) loss of purification capacity due to the alternate aerobic-anoxic conditions, and (b) large amount of excess sludge produced due to the short SRT of only 10 days to 30 days.
Australian Patent Application No. AU2003210073 discusses an improvement tailored to remove the drawback of ‘capacity loss due to the alternate aerobic-anoxic conditions’. To this end, the membrane bioreactor for wastewater purification, comprising a upper compartment with membranes where purification takes place under aerobic condition and lower compartment where anoxic condition prevails in which a mixture of activated sludge and said wastewater circulates between the two compartments is characterized in that the anoxic compartment is placed in direct communication with and below the aerobic compartment. Therefore, an optimum mixing of wastewater and sludge throughout the content of the reactor that enables better continuous contact between bacteria and nutrients and thus avoid the loss of capacity due to intermittent aerobic-anoxic operation mode. However, large amount of excess sludge still needs to be wasted from the bioreactor to operate membrane bioreactor at designed SRT. U.S. Pat. No. US6,086,766 introduces ozone treatment of recycled sludge to achieve zero excess sludge discharge by ozonation of 2.5-3.5 times, preferably 2.8-3.4 times amount of the formed excess sludge in conventional aerobic treatment without the use of membrane separation. However, the energy cost associated with ozone generation is extremely expensive. Focused on the sludge reduction, Australian Patent Application No. AU20032 12695 combines alkaline (e.g., NaGH and Ca(OH)2) and ozonation as a pretreatment step to improve sludge's biodegradability prior to membrane bioreactor. Recently, US Patent Application Publication No. US2005/0023202 employs thermal hydrolysis at temperature 50° C.-100° C. and neutralization for sludge pretreatment to offer more biodegradable feed to subsequent membrane bioreactor. However, both the addition of chemicals and heating up large volume of excess sludge are very costly and impractical for a full-scale wastewater treatment plant.
To date, the design and operation of a membrane bioreactor can hardly takes zero excess sludge (namely, SRT approaches the infinite) as an option, because all the existing membrane modules worldwide are not able to handle extremely high sludge concentrations beyond 10 g/L-20 g/L in term of mixed liquor suspended solids (MLSS) due to serious membrane fouling and frequent membrane cleaning and remarkable energy consumption relating to intensive air scouring and oxygenating. In addition, intensive air scouring deteriorates permeate extraction in both quality and quantity.